Romney swept through Florida, whose Jan. 31 primary election could play a conclusive role in determining which Republican wins the party's nomination to run against Obama in next November's presidential election.

On a visit to Miami, Romney picked up the endorsement of three Cuban-American leaders, which could bolster his support among the party's conservative and Hispanic voters. They are Reps. Ileana Ros-Lehtinen and Mario Diaz-Balart and former Representative Lincoln Diaz-Balart.

At that event, Romney was pressed by reporters for a response to a high-profile Democratic ad unveiled on Monday that accused him of "flip-flopping" - switching positions on key issues for political purposes.

"Bring it on," the former governor of Massachusett said. "We're ready for them."

In Tampa, Romney toured the port and accused Obama of stalling trade deals with South Korea, Colombia and Panama.

The deals, finally approved in October, had been proposed by Republican President George W. Bush but were caught up in partisan wrangling for years. Labor groups had fought them over concerns U.S. jobs would be shipped overseas.

"President Obama put those agreements on hold," Romney said. "Because there were people that supported President Obama who wanted those agreements to be either shelved or slowed down. So trade was used as a political feature, as opposed to something that could create jobs."

Obama's re-election campaign was quick to respond.

Spokeswoman Kara Carscaden said Obama negotiated "smart, fair trade agreements" and "will not sign trade agreements just to sign agreements and will only agree to trade deals that increase jobs and exports for Americans."

Romney is trying to fend off a challenge for the Republican nomination from Newt Gingrich, a former Speaker of the House of Representatives who said in South Carolina on Monday he is the "solid conservative alternative" to the more moderate Romney.

In a sign Romney has work to do to deal with questions about switching positions, Gingrich said when asked about the issue: "I wouldn't lie to the American people. I wouldn't switch my positions for political reasons."

Romney was asked about Gingrich in an interview on the Fox News program "Special Report with Bret Baier" that aired on Tuesday. Romney - who ran Bain Capital, a private equity and leveraged buyout firm, before getting into politics - got in a small dig, calling his rival a product of Washington.

"Speaker Gingrich is a good man. He and I have very different backgrounds. He spent his last 30 or 40 years in Washington. I spent my career in the private sector. I think that's what the country needs right now," he said.

Romney is paying most of his attention to winning New Hampshire's Jan. 10 primary but has an active campaign in Iowa, whose caucuses start the nominating process on Jan. 3. He is hoping a win in New Hampshire will catapult him to a strong finish in South Carolina on Jan. 21 and a victory in Florida.

Romney is having trouble attracting conservatives who worry he is too moderate for their tastes. An example they cite is the healthcare plan he developed for Massachusetts that Obama has said was a model for the U.S. overhaul he pushed through Congress in 2010.

Romney told Fox his continued support for the Massachusetts plan is a sign he is not a political shape-shifter.

"I'm standing by what I did in Massachusetts. I'm not trying to dust it aside. I'm absolutely firm that it was the right thing for our state," he said. "I'll defend that and I understand it has political implications. And if it keeps me from winning a primary, so be it."